Indeed. What would happen to FB if for some reason both Apple and Google simultaneously removed their apps from their respective apps stores? I'm not sure it would destroy FB, but it would be a huge blow.
Push comes to shove, I guess Facebook could launch their own mobile platform too? They already have their own Android phone - it's buried inside the Quest headset. But they probably don't need to, unless Apple/Google disallows the use of Facebook SDK by all the other apps in their app stores. Until then, the sweet sweet surveillance data will keep flowing.